Process for the production of hydrocarbons with low sulfur and mercaptan content

ABSTRACT

Process for desulfurization of a cut containing hydrocarbons comprising sulfur compounds and olefinic compounds comprising at least the following successive steps: a first desulfurization in the presence of hydrogen and a hydrodesulfurization catalyst under conditions leading to a desulfurization rate of said cut strictly higher than 90%; separation of most of the hydrogen sulfide from the effluents resulting from the first desulfurization; a second desulfurization of the effluents, with the hydrogen sulfide removed, resulting from the separation step, in the presence of hydrogen and a hydrodesulfurization catalyst, and under conditions leading to a desulfurization rate of said effluents less than that of the first desulfurization.

[0001] This invention relates to a process for the production of hydrocarbons with low sulfur content. This fraction of hydrocarbons contains an olefin fraction generally higher than 5% by weight and most often higher than 10% by weight. The process makes it possible notably to valorize an entire gasoline cut containing sulfur by reducing the sulfur and mercaptan content of said gasoline cut to very low levels, without reducing the gasoline yield, and while minimizing the reduction in the octane rating during said process. The invention is applicable particularly when the gasoline to be processed is a gasoline from catalytic cracking with a sulfur content greater than 500 ppm by weight or even higher than 1000 ppm by weight, even 2000 ppm by weight, and when the desired sulfur content in the desulfurized gasoline is less than 50 ppm by weight, even 20 ppm by weight or even 10 ppm by weight.

PRIOR ART

[0002] Future specifications for automobile fuels are expected to show a great reduction in the sulfur content of these fuels, notably in gasolines. This reduction is intended to limit notably the sulfur oxide content in the exhaust gases of automobiles. Current specifications for sulfur contents are on the order of 150 ppm by weight and will be reduced in the coming years to achieve contents lower than 10 ppm after a transition to 30 ppm by weight. The evolution of specifications for sulfur content in fuels thus requires the development of new processes for extensive desulfurization of fuels.

[0003] The principal sources of sulfur in gasoline bases are fuels from so-called cracking, and mainly the gasoline fraction resulting from a catalytic cracking process of a residue from atmospheric or vacuum distillation of a crude petroleum. The gasoline fraction resulting from catalytic cracking, which represents an average of 40% of gasoline bases, in fact makes up more than 90% of the amount of sulfur in gasolines. Consequently, the production of low-sulfur gasolines requires a step of desulfurization of gasolines from catalytic cracking. This desulfurization is conventionally performed by one or several steps of placing the sulfurous compounds contained in said gasolines in contact with a hydrogen-rich gas in a process called hydrodesulfurization.

[0004] Further, the octane rating of such gasolines is very strongly tied to their olefin content. The preservation of the octane rating of these gasolines thus requires that the transformation reactions of olefins into paraffins, which are inherent to hydrodesulfurization processes, be limited.

[0005] Further, gasolines have corrosive properties because of the presence of mercaptans. To limit the corrosiveness of gasolines, it is generally necessary greatly to lower the mercaptan content to values at least lower than 10 ppm and ideally 5 ppm. The mercaptans measured in desulfurized gasolines are called recombination mercaptans, i.e., resulting from the addition reaction of hydrogen sulfide (H₂S) produced during the desulfurization step and of the olefins present in the gasoline. The solution usually used to eliminate these mercaptans consists in hydrogenating the olefins present in the gasoline. However, for the reasons already described, this hydrogenation results in a latent octane loss in gasolines from catalytic cracking.

[0006] Numerous solutions have been proposed to selectively eliminate sulfurous compounds in gasolines by limiting undesirable hydrogenation reactions of the olefins, generally evaluated by one skilled in the art in the form of an olefin saturation rate at the exit of the reactor. Among these processes there are processes in which the gasoline is processed in one or two reactors in series without intermediate separation of H₂S. These processes make it possible partially to resolve the problem of the octane and mercaptan index in batches with a sulfur content not exceeding generally 1000 ppm, and for which the desired desulfurization rates are low, typically less than 90%.

[0007] The processing of sulfur-rich gasolines (i.e., containing more than 1000 ppm or even more than 2000 ppm of sulfur) aimed at ultimately reaching sulfur contents less than 50 ppm, even 20 ppm or even 10 ppm, can optionally and preferably require the use of a process comprising hydrodesulfurization in at least two hydrodesulfurization reactors in series, and intermediate elimination of the H₂S formed during the first hydrodesulfurization step. This type of plan is preferably intended to achieve high desulfurization rates, for example rates of 99%, to bring a gasoline with sulfur concentrations on the order of 2000 ppm to sulfur concentrations on the order of 10 ppm.

[0008] For example, patent EP 0755995 proposes a plan consisting of at least two hydrodesulfurization steps and a step for eliminating the H₂S between two hydrodesulfurization reactors. The hydrodesulfurization rate must, at each step, be between 60% and 90%. But such a process does not make it possible to expect to achieve desulfurization rates higher than 99% at industrial scale. To reach a more extensive desulfurization, it would appear necessary to add at least one supplemental step, which greatly limits the economic attractiveness of the process.

[0009] U.S. Pat. No. 6,231,753 proposes the processing of highly sulfurous gasolines with a plan also comprising 2 hydrodesulfurization steps and an intermediate elimination of the H₂S formed. The operating conditions are such that the desulfurization rate and temperature of the gasolines of the second hydrodesulfurization step are greater than those of the first step.

SUMMARY OF THE INVENTION

[0010] Generally, this invention relates to a new process comprising 2 steps of hydrodesulfurization and intermediate elimination of H₂S which simultaneously makes it possible to:

[0011] achieve future specifications for automobile gasolines, i.e., sulfur contents on the order of 30 ppm or even 10 ppm, depending on the country.

[0012] to control the olefin hydrogenation process during said process.

[0013] to limit the loss of octane rating connected with hydrodesulfurization processes,

[0014] to decrease the mercaptan content for a given sulfur and olefin content in desulfurized gasolines.

[0015] More specifically, the invention relates to a desulfurization process for a cut containing hydrocarbons comprising sulfur and olefinic compounds comprising at least the following successive steps:

[0016] a first desulfurization in the presence of hydrogen and a hydrodesulfurization catalyst under conditions leading to a desulfurization rate of said cut strictly higher than 90%,

[0017] a separation of most of the hydrogen sulfide (H₂S) from the effluents resulting from the first desulfurization,

[0018] a second desulfurization of the effluents, with the hydrogen sulfide removed, resulting from the separation step, in the presence of hydrogen and of a hydrodesulfurization catalyst and under conditions leading to a desulfurization rate of said effluents less than that of the first desulfurization.

[0019] Thus, this process of desulfurization proposes a solution for achieving high desulfurization rates, typically higher than 95% and more specifically higher than 99%, while limiting the octane loss through hydrogenation of the olefins, as well as the formation of recombination mercaptans. The result is the production of a gasoline low in sulfur and mercaptans and with a high octane rating.

DETAILED DESCRIPTION OF THE INVENTION

[0020] The invention relates to a process of desulfurization of a cut containing hydrocarbons comprising sulfur and olefinic compounds comprising at least the following successive steps:

[0021] a first desulfurization in the presence of hydrogen and a hydrodesulfurization catalyst under conditions leading to a desulfurization rate of said cut strictly higher than 90%,

[0022] separation of most of the hydrogen sulfide (H₂S) from the effluents resulting from the first desulfurization,

[0023] a second desulfurization of the effluents, with the hydrogen sulfide removed, resulting from the separation step, in the presence of hydrogen and of a hydrodesulfurization catalyst and under conditions leading to a desulfurization rate of said effluents less than that of the first desulfurization.

[0024] In the process according to the invention, preferably at least one of the hydrodesulfurization catalysts comprises at least one element from Group VIII of the periodic table and, more preferably, at least one of the catalysts further has at least one element from group VIB of the periodic table.

[0025] Very preferably, said hydrodesulfurization catalysts include at least one element from Group VIII of the periodic table, selected from the group consisting of nickel and cobalt and at least one element from Group VIB of the periodic table, selected from the group consisting of molybdenum and tungsten.

[0026] Preferably, the first desulfurization of the process according to the invention is performed at a temperature between 250° C. and 350° C., under pressure between 1 and 3 MPa, at an hourly spatial liquid speed between 1 h⁻¹ and 10 h⁻¹ and with an H₂/HC ratio between 50 l/l and 500 l/l, and preferably the second desulfurization is performed at a temperature between 200° C. and 300° C., at a pressure between 1 and 3 MPa, at an hourly spatial liquid speed between 1 h⁻¹ and 10 h⁻¹ and with an H₂/HC ratio between 50 l/l and 500 l/l.

[0027] Preferably, the desulfurization rate of the second desulfurization is strictly higher than 80% and, more preferably, the difference between the desulfurization rates of the first and second desulfurization is at least one percent in absolute value. Such a difference can be obtained notably thanks to a temperature difference and/or in hourly volume speed between the first and second desulfurization, and/or by different catalytic activity of the catalysts used in the first and second hydrodesulfurization step, for example due to a difference in the composition or preparation of these catalysts.

[0028] The process according to the invention can optionally further comprise a supplemental step of selective hydrogenation of the diolefins contained in the cut containing hydrocarbons, said step being performed before the first desulfurization. It can more preferably further comprise a step of making the light sulfurous compounds heavy increasing the boiling point of the light sulfur compounds by combining them with another compound, said step being performed before the first desulfurization, and very preferably further comprise at least one supplemental separation step of said cut into at least two fractions:

[0029] a light fraction containing a small part of the sulfurous compounds,

[0030] a heavy fraction containing most of the sulfurous compounds, at least said heavy fraction then being processed according to the first desulfurization step.

[0031] The process according to the invention is preferably applied to batches such as gasoline cuts resulting from catalytic cracking or from coking a heavy batch containing hydrocarbons, or from steam cracking.

[0032] The invention will be better understood from reading the following embodiment, given purely by way of nonlimiting example.

[0033] According to the preferred but nonobligatory embodiment of the invention that follows, the batch to be desulfurized is preferably processed optionally in a series of reactors for selective hydrogenation of the diolefins (step a) and of making the light sulfurous compounds heavy (step b). The batch thus pre-processed is then distilled and fractionated into at least two cuts (step c): a light gasoline low in sulfur and high in olefins and a heavy gasoline high in sulfur and low in olefins. The light fraction resulting from the three preceding steps generally contains less than 50 ppm of sulfur, preferably less than 20 ppm of sulfur, very preferably less than 10 ppm of sulfur, and generally does not require subsequent processing before being used as a base gasoline. The heavy fraction resulting from the three preceding steps in which most of the sulfur is concentrated is processed according to the process that is the object of this invention. This preferred embodiment offers the advantage of further minimizing the loss of octane because the light olefins with 5 carbon atoms, easily hydrogenated, are not sent to the hydrodesulfurization section.

[0034] Generally, the experimental conditions of these pre-processing steps a), b), and c) are the following:

[0035] 1) Selective Hydrogenation (Step a):

[0036] This optional step of pre-processing the gasoline to be desulfurized is intended to eliminate at least partially the diolefins present in the gasoline. The hydrogenation of dienes is an optional but advantageous step that makes it possible to eliminate practically all the dienes present in the cut to be processed before the hydro-treating. The diolefins are precursors of gums that polymerize in hydrotreatment reactors and thus limit their lifetime.

[0037] This step is generally performed in the presence of a catalyst comprising at least one metal from group VIII, preferably selected from the group formed by platinum, palladium, and nickel, and a support. A catalyst containing 1 to 20% by weight of nickel deposited on an inert support, such as for example aluminum, silicon, silica-aluminum, a nickel aluminate or a support containing at least 50% aluminum will be used. This catalyst generally operates at a pressure of 0.4 to 5 MPa, at a temperature of 50 to 250° C., with an hourly spatial liquid speed of 1 h⁻¹ to 10 h⁻¹. Another metal from group VIb can be combined to form a bimetallic catalyst, such as for example molybdenum or tungsten. This metal from group VIb, if it is combined with the metal from group VIII, will be deposited at a rate of 1% by weight to 20% by weight on the support.

[0038] The choice of operating conditions is particularly important. Most generally the process is done under pressure in the presence of an amount of hydrogen that is slightly higher than the stoichiometric value necessary to hydrogenate diolefins. The hydrogen and the batch to be processed are injected as ascending or descending currents in a reactor that is preferably a fixed catalyst bed. The temperature is most generally between 50 and 300° C., preferably between 80 and 250° C., most preferably between 120 and 210° C.

[0039] The pressure is selected so as to be sufficient to maintain more than 80%, and preferably more than 95% by weight of the gasoline to be processed in liquid phase in the reactor; it is more generally 0.4 to 5 MPa and preferably higher than 1 MPa. An advantageous pressure is between 1 and 4 MPa, those limits included.

[0040] Under these conditions, the spatial speed is on the order of 1 to 12 h⁻¹, preferably on the order of 4 to 10 h⁻¹.

[0041] The light fraction from the gasoline cut from catalytic cracking can contain up to several % by weight of diolefins. After hydrogenation, the diolefin content is reduced to less than 3000 ppm, even to less than 2500 ppm and better to less than 1500 ppm. In certain cases, a diene content less than 500 ppm can be obtained. The diene content after selective hydrogenation can even be reduced in certain cases to less than 250 ppm.

[0042] Simultaneously with the selective hydrogenation reaction of the diolefins, there is an isomerization of the double external olefin bond leading to the formation of internal olefins. This isomerization results in the formation of olefins that are more resistant to hydrogen saturation and leads to a slight gain in the octane number (or a compensation of the octane number due to the slight loss of olefin). This is probably due to the fact that internal olefins have an octane rating generally higher than that of terminal olefins.

[0043] According to an embodiment of the invention, the hydrogenation step of dienes is performed in a catalytic hydrogenation reactor that comprises at least one catalytic reaction zone through which the entire batch, and the amount of hydrogen necessary to perform the desired reactions, generally pass.

[0044] 2) Making the Light Sulfurous Compounds Heavy (Step b):

[0045] This optional step consists in transforming the saturated light sulfur compounds, i.e., the compounds whose boiling temperature is less than that of thiophene, into saturated sulfur compounds whose boiling temperature is higher than that of thiophene. These light sulfurous compounds are typically mercaptans with 1 to 5 carbon atoms, CS₂, and sulfurs with 2 to 4 carbon atoms. This transformation is preferably performed on a catalyst comprising at least one element from group VIII (groups 8, 9, and 10 of the new periodic table) on an aluminum-type, silica or aluminum silica, or nickel aluminate support. The choice of catalyst is made notably so as to promote the reaction between the light mercaptans and the olefins, which leads to mercaptans or sulfurs with boiling temperatures higher than thiophene.

[0046] This optional step can be optionally performed at the same time as step a), on the same catalyst. For example, it can be particularly advantageous to operate, during hydrogenation of the diolefins, under conditions such that at least some of the compounds in the form of mercaptans are transformed.

[0047] In this case, temperatures are generally between 100 and 300° C. and preferably between 150 and 250° C. The H₂/batch ratio is adjusted between 1 and 20 liters per liter, preferably between 3 and 15 liters per liter. The spatial speed is generally between 1 and 10 h⁻¹, preferably between 2 and 6 h⁻¹, and the pressure is between 0.5 and 5 MPa, preferably between 1 and 3 MPa.

[0048] 3) Separation of the Gasoline into at Least Two Fractions (Step c)

[0049] This step is optional. When it is performed after steps a) and b), it makes it possible to produce a light, desulfured gasoline containing most often less than 50 ppm of mercaptans. During this step, the gasoline is fractionated into at least two fractions:

[0050] a light fraction with a limited residual sulfur content, preferably less than about 50 ppm, preferably less than about 20 ppm, very preferably less than about 10 ppm, and making it possible to use this cut without performing other treatment(s) intended to reduce its sulfur content.

[0051] a heavy fraction in which most of the sulfur, i.e., all of the sulfur initially present in the batch and that is not in the light gasoline, is concentrated.

[0052] This separation is performed preferably using a conventional distillation column. This fractionation column must make it possible to separate a light gasoline fraction containing a low sulfur fraction and a heavy fraction containing preferably most of the sulfur initially present in the initial gasoline.

[0053] The light gasoline obtained after the separation generally contains at least all the olefins with 5 carbon atoms, preferably compounds with 5 carbon atoms and at least 20% of olefins with six carbon atoms. Generally, this light fraction obtained after steps a) and b)have a low sulfur content, i.e., it is generally not necessary to treat the light cut before using it as fuel.

[0054] The gasoline processed using the variant of the process according to the invention, which is described below, is a cracked gasoline resulting directly from the cracking or pretreating unit according to at least one of steps a), b), or c) described above.

[0055] The process according to the invention comprises two desulfurization steps d) and f) performed in two separate reaction zones, as well as a step e) for H₂S separation between the two hydrodesulfurization zones.

[0056] The first hydrodesulfurization step (step d) consists in passing the gasoline to be treated over a hydrodesulfurization catalyst, in the presence of hydrogen, at a temperature between 250° C. and 350° C., preferably between 270° C. and 320° C. and at a pressure between 1 and 3 MPa, preferably between 1.5 and 2.5 MPa. The spatial liquid speed is generally between 1 h⁻¹, and 10 h⁻¹, preferably between 2 h⁻¹ and 5 h⁻¹, the H₂/HC ratio is between 50 liters/liter (1/1) and 500 l/l, preferably between 100 l/l and 400 l/l, more preferably between 150 l/l and 300 l/l. The H₂/HC ratio is the ratio between the hydrogen throughput at 1 atmosphere and 0° C. and the hydrocarbon throughput. Under these conditions, the reaction takes place in the gaseous phase. The desulfurization rate achieved during this step is strictly higher than 90%, i.e., for example, a gasoline initially containing 2000 ppm of sulfur will be transformed into a gasoline containing less than 200 ppm of sulfur. The operating conditions during this step are thus adjusted depending on the characteristics of the batch to be treated so as to achieve a desulfurization rate strictly higher than 90%, preferably higher than 92%, and very preferably higher than 94%. The effluents resulting from this first hydrodesulfurization step are partially desulfurized gasoline, residual hydrogen, and H₂S produced by decomposition of sulfurous compounds.

[0057] This step is followed by a step (step 3) of separating most of the H₂S from the other effluents. This step is intended to eliminate at least 80% and preferably at least 90% of the H₂S produced during step d). The elimination of the H₂S can also be achieved in different ways, known for the most part by one skilled in the art. For example, there is absorption of H₂S by a mass of metallic oxide, selected preferably from the group consisting of zinc oxide, copper oxide, or molybdenum oxide. This absorbent mass can preferably be regenerated and will be able to be regenerated continuously or discontinuously by, for example, thermal processing in an oxidizing or reducing atmosphere. The adsorbent mass can be used in a fixed or mobile bed. Another more conventional method consists in cooling the effluent of step d) to produce a liquid and a gas that are rich in H₂ and H₂S. The H₂S can be separated from the H₂ by means of a washing unit with amines, whose operation is well known to one skilled in the art.

[0058] A second desulfurization step f) is intended to achieve extensive desulfurization of the gasoline resulting from step e) to the desired sulfur content. This step consists in making the gasoline resulting from step e), mixed with hydrogen, pass over a hydrodesulfurization catalyst at a temperature between 200° C. and 300° C., preferably between 240° C. and 290° C., at a pressure between 1 and 3 MPa, preferably between 1.5 and 2.5 MPa. The liquid spatial speed is generally between 1 h⁻¹ and 10 h⁻¹, preferably between 2 h⁻¹ and 8 h⁻¹, the H₂/C ratio is between 50 liters/liter (l/l) and 500 l/l, preferably between 100 l/l and 400 l/l, and most preferably between 150 l/l and 300 l/l. Under these conditions, the reaction takes place in the gaseous phase. The mixture of gasoline and hydrogen processed during this step contains less than 100 ppm of H₂S and preferably less than 50 ppm of H₂S. The operating conditions of this step are such that the desulfurization rate makes it possible to achieve the required sulfur content while maintaining a desulfurization rate less than that of the first step. The batch to be processed during this step is much less sulfurous than the initial batch, and the desired desulfurization rates are much less. Consequently, the necessary catalyst volumes and the operating temperatures are likewise much lower. For example, the VVH (hourly volumic speed) of desulfurization step f) can be 1.5 times greater than the VVH of step d), and/or the temperature of desulfurization step f) can, for example, be at least 10° C. and advantageously at least 20° C. less than that of step d). Most often, the catalyst volumes are fixed in industrial units, the desulfurization rate of step f) is then adjusted mainly by temperature. In any case, said difference can be adjusted without going beyond the scope of the invention, by any means known to act on the desulfurization rate of each step d) or f), for example using, for step f), a catalyst that is less active than that of step d). The difference in activity between the catalysts of steps d) and f) can, for example, be obtained by using, for step f), a catalyst containing a lower quantity of metals or a support with a smaller specific surface area compared to the catalyst of step d). Another solution can consist in using a partially deactivated catalyst for step f).

[0059] The operating conditions during step f) are thus adjusted depending on the characteristics of the batch to be processed to achieve a desulfurization rate most often strictly greater than 80%, preferably greater than 85%, and very preferably greater than 92%, or even greater than 95%.

[0060] The difference between the desulfurization rates of the first and second step of hydrodesulfurization is generally greater than 1%, preferably greater than 2%, and very preferably greater than 3% in absolute value.

[0061] Surprisingly, it was found by the applicant that such constraints on the respective desulfurization rates of steps d) and f) make it possible to minimize the mercaptan content of the gasoline produced and thus to make any subsequent step of sweetening the gasoline optional or less constraining.

[0062] According to a variant embodiment of this process, it is also possible to inject fresh hydrogen into the second hydrodesulfurization reactor, to separate the hydrogen from the gasoline produced and to inject this hydrogen, which generally contains less than 200 ppm of H2S, into the first hydrodesulfurization step.

[0063] The catalysts used during steps d) and f) comprise at least one element from group VIII and/or at least one element from group VIB on an appropriate support.

[0064] The content of metal from group VIII, expressed as an oxide, is generally between about 0.5 and 15% by weight, preferably between 1 and 10% by weight. The content of metal from group VIB is generally between 1.5 and 60% by weight, preferably between 3 and 50% by weight.

[0065] The element of group VIII, when it is present, is preferably cobalt, and the element of group VIB, when it is present, is generally molybdenum or tungsten. The support of the catalyst is usually a porous solid, such as, for example, an aluminum, a silica aluminum, or other porous solids, such as, for example, magnesium, silica or titanium oxide, alone or mixed with aluminum or silica aluminum. To minimize hydrogenation of the olefins present in heavy gasoline, it is advantageous preferably to use a catalyst in which the molybdenum density, expressed in % by weight of MoO3 per unit of surface, is greater than 0.07 and preferably greater than 0.10. The catalyst according to the invention preferably has a specific surface area less than 190 m²/g, more preferably less than 180 m²/g, and very preferably less than 150 m²/g.

[0066] After introducing the element(s) and optionally formatting the catalyst (when this step is performed on a mixture that already contains the base elements), the catalyst is in a first stage of activity. This activation can correspond to either an oxidation and then a reduction, or to a direct reduction, or to a calcination only. The calcination step is generally performed at temperatures going from about 100 to about 600° C., preferably between 200 and 450° C., with air flowing through.

[0067] The catalyst is preferably used at least partially in its sulfurous form. The introduction of sulfur can be done before or after any activation step, i.e., calcination or reduction. Preferably, no oxidation step of the catalyst is performed once the sulfur or a sulfurous compound has been introduced onto the catalyst. The sulfur or sulfurous compound can be introduced ex situ, i.e., outside the reactor where the process according to which the invention is performed, or in situ, i.e., inside the reactor used for the process according to the invention. In this latter case, the catalyst is preferably reduced under the above-described conditions, then sulfured by passage of a batch containing at least on sulfurous compound which, once decomposed, leads to the fixation of sulfur on the catalyst. This batch can be gaseous or liquid, for example hydrogen containing H₂S, or a liquid containing at least one sulfurous compound.

[0068] The significance and the advantages of this invention are obvious from comparison of examples 1 and 2 according to the prior art and example 3, according to the invention.

EXAMPLE 1 According to the Prior Art

[0069] Example 1 relates to a desulfurization process without intermediate elimination of H₂S and with a hydrodesulfurization step.

[0070] A catalyst A for hydrodesulfurization is obtained by impregnation “without excess solution” of a transition aluminum available in the form of small balls with a specific surface area of 130 m2/g and pore volume of 0.9 ml/g, with an aqueous solution containing molybdenum and cobalt in the form of ammonium heptamolybdate and cobalt nitrate. The catalyst is then dried and calcined under air at 500° C. The cobalt and molybdenum content of this sample is 3% CoO and 10% MoO3.

[0071] 100 ml of catalyst A is placed in a fixed bed, tubular hydrodesulfurization reactor. The catalyst is first sulfured by processing for 4 hours under a pressure of 3.4 MPa at 350° C., in contact with a batch consisting of 2% sulfur in the form of dimethyl disulfide in n-heptane.

[0072] The batch processed is a gasoline from catalytic cracking whose initial boiling point is 50° C. and final boiling point is 225° C. Its sulfur content is 2000 ppm by weight and its bromine index (iBr) is 69 g/100 g, which corresponds to about 36% by weight of olefins.

[0073] This batch is processed on catalyst A, under a pressure of 2 MPa bar, an H₂/HC ratio of 300 l/l and a VVH of 2 h⁻¹. Table 1 shows the influence of the temperature on the desulfurization and olefin saturation rates. TABLE 1 Sulfur Mer- Saturation Tem- content of captan Desulfuri- iBr Of Rate of perature Desulfured Content zation Desulfurized Olefins (° C.) Gasoline (ppm) (ppm) Rate (%) Gasoline (HDO) 310 41 32 97.9 20.3 70.6 320 23 20 98.8 14.7 78.7 330 12 11 99.4 10 85.5

[0074] The operating conditions required to achieve 10 ppm of sulfur with this type of highly sulfurous gasoline are high temperature (>310° C.) and low VVH (2 h⁻¹). Under these conditions, it is possible to achieve desulfurization rates higher than 99%, but the olefin saturation rate then becomes very high (greater than 85%), which acts negatively on the octane rating.

EXAMPLE 2 According to the Prior Art

[0075] Example 2 relates to a desulfurization process with two hydrodesulfurization steps and intermediate elimination of the H₂S formed, according to the prior art.

[0076] Catalyst A is used under gentler conditions than those of example 1. According to the prior art, the desulfurization rate of the second hydrodesulfurization step is higher than that of the first step. The batch processed is the same as the batch of example 1.

[0077] The batch is sent into the reactor of example 1 on catalyst A mixed with hydrogen. The operating temperature is 285° C. Other operating conditions are specified in table 2. The effluents coming out of the reactor contain 239 ppm of sulfur. They are cooled and stripped so as to separate the hydrogen and the H₂S from the hydrocarbon phase. The stripped effluents are then reinjected into the reactor loaded with catalyst A mixed with fresh hydrogen, according to the operating conditions of said second step indicated in table 2. The batch throughput was multiplied by 1.5 with respect to the throughput of the first step. The experimental device used has an online sulfur analyzer which makes it possible continuously to measure the sulfur content of the effluents. The reactor temperature during the second step was adjusted so as to produce a gasoline containing 10 ppm of sulfur. TABLE 2 First step Temperature 1 ° C. 285 VVH 1 h−1 4 Pressure 1 bar 20 H2/HC1 l/l 300 Sulfur at exit ppm 239 iBr at exit 49.9 HDS % 88.1 Second step Temperature 2 ° C. 292 VVH 2 h−1 6 Pressure 2 bar 20 H2/HC2 l/l 300 Sulfur at exit ppm 10 Mercaptans at exit ppm 9 iBr at exit 36.9 HDS % 96.0 Overall HDS % 99.5 Overall HDO % 46.6

[0078] This example shows that the process comprising two hydrodesulfurization steps with intermediate elimination of H₂S is much more selective than the process with one step used in example 1. Indeed, the gasoline produced in example 2 has the same sulfur content as the gasoline of example 1, but the olefin saturation rate (HDO) here is 46.7%, compared to 85.5% for example 1. The process used in example 2 thus makes it possible to minimize the processes leading to olefin saturation during hydrodesulfurization.

EXAMPLE 3 According to the Invention

[0079] In this example, the desulfurization rate of the first step is, in contrast to example 2, higher than that of the second step.

[0080] The batch processed is the same as in examples 1 and 2.

[0081] The batch is sent into the above-described reactor on a catalyst A mixed with hydrogen. The operating temperature is 300° C. The other operating conditions are specified in table 3. The effluents exiting the reactor contain, respectively, 117 ppm of sulfur. The operating mode is the same as for example 2: the effluents are cooled and stripped so as to separate the hydrogen and the H₂S from the hydrocarbon phase, which is reinjected into the reactor loaded with catalyst A, mixed with fresh hydrogen, according to the operating conditions indicated in table 3. The batch throughput was multiplied by 1.5 with respect to the throughput of the first desulfurization step. As in example 2, the temperature was adjusted so as to finally recover, at the exit of the reactor, a gasoline containing 10 ppm of sulfur. TABLE 3 First step Temperature 1 ° C. 300 VVH 1 h⁻¹ 4 Pressure 1 MPa 2 H₂/HC1 l/l 300 Sulfur at exit ppm 117 iBr at exit 43.4 HDS % 94.2 Second step Temperature 2 ° C. 264 VVH 2 h⁻¹ 6 Pressure 2 MPa 2 H₂/HC2 l/l 300 Sulfur at exit ppm 10 Mercaptans at exit ppm 6 iBr at exit 36.8 HDS % 91.2 Overall HDS % 99.5 Overall HDO % 46.7

[0082] The process according to the invention, i.e., use of a desulfurization rate of the first step greater than that of the second step, makes it possible to achieve the same sulfur content in desulfurized gasoline, as well as the same olefin saturation rate as the prior art, illustrated by example 2. But the effluent produced using the process according to the invention has 30% less mercaptans than the gasoline resulting from example 2. The use of the process according to this invention thus makes it possible not only to greatly limit olefin saturation but also to greatly decrease the mercaptan content and thus the corrosiveness of the gasoline produced.

[0083] It is to be noted that the adjective “sulfurous” used in this application is not limited to tetravalent forms of sulfur, but instead is intended to include all forms of sulfur-containing compounds and elemental sulfur.

[0084] The preceding examples can be repeated with similar success by substituting the generically or specifically described reactants and/or operating conditions of this invention for those used in the preceding examples.

[0085] From the foregoing description, one skilled in the art can easily ascertain the essential characteristics of this invention and, without departing from the spirit and scope thereof, can make various changes and modifications of the invention to adapt it to various usages and conditions.

[0086] The entire disclosure of all applications, patents and publications, cited herein and of corresponding French application No. 02/04.108, filed Mar. 29, 2003 is incorporated by reference herein. 

1. Desulfurization process for a cut containing hydrocarbons comprising sulfur and olefinic compounds and having at least the following successive steps: a first desulfurization in the presence of hydrogen and a hydrodesulfurization catalyst under conditions leading to a desulfurization rate of said cut strictly higher than 90%, separation of most of the hydrogen sulfide (H₂S) from the effluents resulting from the first desulfurization, a second desulfurization of the effluents, with the hydrogen sulfide removed, resulting from the separation step, in the presence of hydrogen and of a hydrodesulfurization catalyst and under conditions leading to a desulfurization rate of said effluents less than that of the first desulfurization.
 2. Process according to claim 1, wherein at least one of the hydrodesulfurization catalysts has at least one element from group VIII of the periodic table.
 3. Process according to claim 2, wherein at least one of the catalysts further has at least one element from group VIB of the periodic table.
 4. Process according to claim 1, comprising at least one element from group VIII of the periodic table selected from the group consisting of nickel and cobalt and at least one element from group VIB from the periodic table selected from the group consisting of molybdenum and tungsten.
 5. Process according to one of the preceding claims, wherein the first desulfurization is performed at a temperature between 250° C. and 350° C., under pressure between 1 and 3 MPa, at an hourly spatial liquid speed between 1 h⁻¹ and 10 h⁻¹, and with an H₂/HC ratio between 50 l/l and 500 l/l.
 6. Process according to one of the preceding claims, wherein the second desulfurization is performed at a temperature between 200° C. and 300° C., under pressure between 1 and 3 MPa, at an hourly spatial liquid speed between 1 h⁻¹ and 10 h⁻¹ and with an H₂/HC ratio between 50 l/l and 500 l/l.
 7. Process according to one of the preceding claims, wherein the desulfurization rate of the second desulfurization is strictly greater than 80%.
 8. Process according to one of the preceding claims, wherein the difference between the desulfurization rates of the first and the second desulfurization is at least one percent in absolute value.
 9. Process according to one of the preceding claims, further comprising a supplementary step of selective hydrogenation of the diolefins contained in the cut, said step being performed before the first desulfurization.
 10. Process according to one of the preceding claims, further comprising a supplementary step of making the light sulfurous compounds heavy, said step being performed before the first desulfurization.
 11. Process according to one of the preceding claims, further comprising at least one supplementary separation step for said cut into at least two fractions: a light fraction containing a small amount of the sulfurous compounds, a heavy fraction containing most of the sulfurous compounds, at least said heavy fraction then being processed according to the first desulfurization step.
 12. Application of the process according to the preceding claims to gasolines resulting from catalytic cracking or coking of a heavy batch containing hydrocarbons or steam cracking. 